An electrochemical device comprised of cathode and anode electrodes physically exposed to an electrolyte may benefically be used to convert between chemical and electrical energies. Housing means enclose these electrode and electrolyte components, and may even seal them from the atmosphere.
Batteries, fuel cells and capacitors are but a few such specific electrochemical devices to which this invention relates.
As the electrical capacity in terms of voltage and/or amperage of each pair of cathode and anode electrodes (or cell) is generally small, many separate pairs of cathode and anode electrodes or cells may be used in a single housing means. Current collectors are generally used to electrically interconnect the cells, in parallel and/or in series, to provide usable voltage and amperage outputs at exposed terminals on the electrochemical device.
The electrochemical device performs usable work when ions pass between the electrodes of each cell via the electrolyte, and when electrons concurrently pass through each cell via the electrodes. The generated voltage per cell is predetermined by the electrochemical reaction of the component materials used, and the generated amperage and/or power available is dependent on the configurations and masses of these active components.
The specific output energy of the device may be provided in terms of watts-hours per device weight, and the specific output power of the device may be provided in terms of watts per device weight. Output values of existing electrochemical devices are typically small fractions of theoretically possible output values, because of internal resistances and other inefficiencies.
The resistance to ion-conduction between the electrode elements is one major source for internal power loss. Such resistance, R, may be theoretically determined with the expression EQU R=.rho.l/A,
where:
".rho." is the impedance value of the electrolyte; PA1 "l" is the thickness of the electrolyte; and PA1 "A" is the interfacial contact area between the electrode elements and electrolyte.
The ion-impedance value, .rho., is not easily subject to modification and is not effective as a design parameter. Designers of electrochemical devices thus strive to reduce the electrolyte thickness "l", and to increase the interfacial contact area "A" between the electrode elements and the electrolyte.
Different configurations of the cathode and anode electrodes, electrolyte separation, and the current collectors are disclosed in the following patents.
U.S. Pat. No. 1,510,364 zig-zagged a cathode electrode band to define separate compartments for holding electrolyte, and inserted elongated rod-like anode electrodes into the electrolyte spaced from the cathode electrode. The interfacial contact area "A" effectively is less than the overall surface area of the anode rods, as some rods oppose one another rather than the cathode.
U.S. Pat. Nos. 2,157,629 and 2,851,509 each generally zig-zagged a folded separator band to define opposing compartments for holding and isolating plate-like cathode and anode electrodes, with electrolyte engulfing all of these components. U.S. Pat. No. 4,029,855 formed each cell with C-shaped electrodes and a Z-shaped separator sandwiched therebetween. U.S. Pat. No. 4,048,397 folded a separator band having electrically conductive surfaces, and sandwiched separate sets of respective plate-like cathode and anode electrodes between the separate oppositely facing folds. U.S. Pat. Nos. 2,665,325; 3,410,726; 4,664,989 and 4,668,320 illustrated "jellyroll" cell constructions each formed by coiling a preformed assembly of cathode and anode electrodes and a separator on itself, to yield a right cylindrically shaped electrochemical device. These cell arrangements, with the face-to-face electrodes and sandwiched electrolyte and separator structures, increased the interfacial contact area "A" between the electrodes.
However, the very breadth of the facing electrodes and sandwiched electrolyte and separator, raise another cause of concern, namely each's structural sufficiency during assembly and during operation, to maintain and support the electrode elements physically separated. This includes withstanding thermal expansion and contraction forces of the cell components during operational temperature changes. Increasing the thickness of the sandwiched electrolyte and separator to provide needed strength and/or durability also increases the ion-conducting electrolyte thickness "l", offsetting benefits obtained by increased interfacial contact area "A".
Current collection means used in these cell arrangements add significant weight, and thus reduced specific cell energy and power outputs. For example, isolated conductors are generally connected to the electrodes and routed along extended paths independently of the electrodes to the external terminals. These conductors must carry the full cell current, and thus must be of sufficient mass and cross-section to keep internal resistance manageably low.
Also, these cell arrangements provide electrodes of limited size and/or thickness, limiting the quantities of usable electrode materials and thus limiting maximum cell storage energy and/or operating cycle-life (for recharagable cells).
The dilemma of these designs is that power gains obtained by increasing the interfacial electrode area "A" across the electrolyte generally are typically offset by increased electrolyte thickness "l", and the weight and volume of the current collectors reduce specific energy and power outputs. Power can be increased, but only at the expense of reduced energy storage capacity per weight and volume and at increased costs due to needed additional hardware. High interfacial area "A" of the spirally wound "jellyroll" configuration merely trades off usable power against the energy density; but minimum separator thickness is needed for cell durability and cycle-life.
Existing bipolar cell arrangements do not escape this power and energy trade off dilemma; nor do fuel cell electrochemical devices.
Fuel cell designers also attempt to maximize interfacial area for increased power density, and the all-ceramic solid oxide fuel cell is promising. Interfacial thermal expansion problems associated with the 1000 degree C. operating temperatures needed to achieve ion conduction via the solid remain a design concern, but the use of thin layered components seems promising, so long as the layered components bond together and have matched thermal expansion coefficients. It would seem likely that if electrolyte layers can be fabricated thin enough in forming the device, electrolyte materials of marginal ion-conductivity could possibly even be used as a viable option; and the reduced resistance may well even lower the overall operating temperatures of the device.
Fuel cell designers moreover must effectively manifold fuel, oxidant and exhaust gases to and from the electrochemical interfaces, to yield the production of the electrical and thermal energies. The gas flow, turbulence, and exchange rates in part determine the chemical activity of the fuel and oxidant gases at the interfaces, and the resulting outputs and efficiencies. Fuel cell voltage decreases rapidly as chemical activity of the fuel becomes less than 5% of the fuel gas; whereby flow rate designs typically attempt to yield 80-90% fuel consumption. Nonetheless, overall energy efficiency may be reduced because of large pressure drops between the gas flow inlets and outlets.